1. Field of the Invention
The present invention relates to an antenna device for use in a near field radio communication system or the like and to an electronic apparatus including the antenna device.
2. Description of the Related Art
In radio frequency identification (REID) of the 13.56 MHz range, such as near field communication (NFC), typically, an integrated circuit (IC) chip for REID and matching element are mounted on a circuit board, an antenna is attached to an inner surface of a terminal housing made of resin, and the IC chip for REID and antenna are connected to each other with a spring pin or the like interposed therebetween in a direct current manner.
Recently, wireless communication devices, such as cellular phone terminals, have become thinner and thinner, and the use of metals in a wide area of a housing has increased to solve a shortage of strength created by the reduction in thickness. However, the metallic portion in the housing shields an antenna incorporated in the terminal, and this causes the problem that communication with a device on a partner side is not established.
Thus, as in Japanese Unexamined Patent Application Publication No. 2011-97657, there is proposed an antenna device having a structure in which a metal plate with a wider area than an antenna coil is placed in the vicinity of (magnetic-field coupled to) the antenna coil and the metal plate is used as a radiator.
Problems to be solved in the antenna device illustrated in Japanese Unexamined Patent Application Publication No. 2011-97657 are described below.
The antenna device described in Japanese Unexamined Patent Application Publication No. 2011-97657 includes a coil conductor and a conductive member magnetic-field coupled to the coil conductor. The conductive member has a conductor aperture and a slit extending from the conductor aperture to an edge portion of the conductive member. The coil conductor is arranged such that its coil aperture portion overlaps the conductor aperture in the conductive member. When a current flows through the coil conductor, a current flows through an outer edge portion of the conductive member in the same direction as that of the current flowing through the coil conductor, whereas a current flows through the peripheral area (inner edge) of the conductor aperture in the vicinity of the coil conductor in a direction opposite to that of the current flowing through the coil conductor. The presence of the area where the current flows in the opposite direction leads to a reduced inductance of the antenna, which results in the problem of degrading communication characteristics. In addition, the strength of an induced current largely varies depending on variations in the position where the coil conductor and conductive member are attached and in the distance between the coil conductor and conductive member when they are attached, and thus there is a problem in that the inductance value tends to vary.